1. Field of the Invention
The present invention relates to a sheet carrying apparatus to carry, for example, a recording paper, or the original for use in a printer, a copying machine, facsimile, a duplicating apparatus, or the like.
2. Description of the Prior Art
FIG. 49 is a perspective view showing an essential part of a sheet carrying apparatus for a conventional color thermal printer disclosed in Japanese Patent Publication (Kokai) No. 4-10965. FIG. 50 is a side view thereof. In FIG. 49, an ink sheet 6 and an ink sheet carrying portion are omitted. In FIGS. 49 and 50, reference numeral 1 means a sheet carrying roller, 2 is first pulleys, 3 is timing belts, 4 is second pulleys, 5 is third pulleys, 6 is an ink sheet, 7 is a sheet supply portion, 9 is a thermal head, and 10 is a clamp mechanism to clamp a distal end of a sheet. The clamp mechanism is, for example, a clamper. Reference numerals 11 and 12 mean drive motors, 13 is a torque limiter, 15 is a supply roll, 16 is a winding roll, 17 is a drive motor, 18 and 19 are torque limiters, and 30 is the sheet. The two first pulleys 2, the two timing belts 3, the two second pulleys 4, and the two third pulleys 5 are respectively disposed on the right and left sides with respect to a sheet center line in a sheet carrying direction.
When the sheet 30 is supplied from the sheet supply portion 7, and the distal end thereof is inserted into the clamper 10, the clamper 10 is closed by an unillustrated clamper opening and closing mechanism so that the clamper 10 can clamp the distal end of the sheet 30. The clamper 10 includes a bridge 10a and a holder 10b mounted to the bridge 10a. The bridge 10a is interposed between the right and left timing belts 3 to extend in a direction perpendicular to the sheet carrying direction. The distal end of the sheet 30 is clamped by the bridge 10a and the holder 10b.
The timing belts 3 are tensed through the first pulleys 2, the second pulleys 4, and the third pulleys 5. A shaft to which the second pulleys 4 are secured is coupled with the drive motor 12 through the torque limiter 13. The clamper 10 mounted to the timing belts 3 is moved by forward rotation of the drive motor 12 in a direction shown by the arrow A while the clamper 10 clamping the sheet 30. Unless the torque limiter 13 is slipped, a speed V.sub.2 of the clamper is determined by a rotational speed of the drive motor 12. When the sheet 30 clamped by the clamper 10 reaches the sheet carrying roller 1, the thermal head 9 is brought into pressure contact with the sheet 30 by an unillustrated mechanism, and the ink sheet 6 and the sheet 30 are held between the sheet carrying roller 1 and the thermal head 9.
The sheet carrying roller 1 is coupled with the drive motor 11. Forward rotation of the drive motor 11 can carry the ink sheet 6 and the sheet 30. The ink sheet 6 is supplied from the supply roll 15 to pass through the sheet carrying roller 1 and the thermal head 9, and is wound up by the winding roll 16. The winding roll 16 is coupled with the drive motor 17 through the torque limiter 18. Rotation of the drive motor 17 rotates the winding roll 16. Further, the supply roll 15 is coupled with the torque limiter 19.
In the above structure, while the sheet 30 is clamped by the clamper 10, the sheet 30 is circularly carried through the first pulleys 2, the third pulleys 5, and the second pulleys 4 so as to return to an original position. During the operation, the thermal head 9 is brought into pressure contact with the sheet 30 through the ink sheet 6, thereby transferring print color material applied onto the ink sheet 6 to the sheet 30. In particular, in case of a color thermal printer, the transferring operation is repeated three or four times, and the ink sheet 6 is exchanged to provide different colors for each transfer, resulting in formation of a color image.
At a time of print, as described above, the sheet 30 is held together with the ink sheet 6 between the sheet carrying roller 1 and the thermal head 9. Therefore, the sheet 30 is carried at a constant speed V.sub.1 which is determined by a rotational speed of the sheet carrying roller 1. Here, the speed V.sub.2 of the clamper 10 is set to be continuously faster than the speed V.sub.1. Hence, during the print operation, the torque limiter 13 is slid to absorb a difference between the speeds V.sub.1 and V.sub.2. When the torque limiter 13 is slid, predetermined torque determined by the torque limiter 13 is transmitted to the clamper 10 through the second pulleys 4 and the timing belts 3. That is, at the time of print, the clamper 10 tenses the sheet 30 with tensile strength according to the predetermined torque. Further, a speed V.sub.3 of the ink sheet is set to be continuously faster than the speed V.sub.1. Thus, as in the clamper 10, at the time of print, tensile strength determined by diameters of the torque limiter 19 and the winding roll 16 is applied to the ink sheet 6 on the winding side. In addition, tensile strength determined by a torque value of the torque limiter 19 and a diameter of the supply roll 15 is applied to the ink sheet 6 on the supplying side. Each of the tensile strength is applied to carry the ink sheet 6 without wrinkling the ink sheet 6.
However, in order to print on the sheet 30 at a predetermined position, it is necessary to finely control, for example, pressure contacting forces in members, and tensile strength of the sheet 30 and the ink sheet 6 such that the sheet 30 is carried in a predetermined carrying direction at a predetermined carrying speed. For example, when a lateral pressure contacting force of the thermal head 9 to the sheet carrying roller 1 is offset, the carrying speed of the sheet 30 tends to become larger in a direction from a position with a low pressure contacting force to a position with a high pressure contacting force. As a result, there is a problem in that the sheet 30 is carried in a state in which the sheet 30 is deviated from the predetermined carrying direction, that is, so-called skew is caused. The skew of the sheet 30 may cause reduced image quality because the image can not be printed on the sheet 30 at the predetermined position.
Further, when a diameter of the sheet carrying roller 1 is increased due to thermal expansion, for example, if a higher gray level image is printed exclusively on a single side in a horizontal scanning direction, the skew is caused. Further, when the diameter of the sheet carrying roller 1 is increased due to thermal expansion, if the higher gray level image is printed through an entire print area, there is another problem in that an amount of carriage of the sheet 30 is deviated from a regular value. That is, the whole amount of carriage is increased so that a predetermined print length can not be obtained.
Since a rubber roller is typically used as the sheet carrying roller 1, wear is caused in the sheet carrying roller 1 due to long-term use so as to decrease its diameter. Hence, there is still another problem in that the print length becomes shorter than that of a predetermined value in process of time.
Axial chattering may be generated in the respective pulleys 2, 4, and 5 at which the timing belts 3 are wound. Thus, there is a further problem in that the clamper 10 secured to the timing belts 3 is deviated by this chattering in the horizontal scanning direction, and the sheet 30 is translated (hereinafter referred to as shifted) in the horizontal scanning direction.
When only a partial distal end of the sheet 30 is slightly released from the clamper 10, the sheet 30 is carried in a slightly inclined state. Thus, there is a further problem in that an amount of shift is more increased at a later end of the sheet.
Further, in the sheet 30, the skew, a deviation of the amount of carriage, and the shift may concurrently be generated. In such a case, it is impossible to overcome the above problems by only control in view of mechanism.
In particular, in case of the thermal color printer, the skew of the sheet, the variation in the amount of carriage in the carrying direction, and the shift in the horizontal scanning direction may be generated during print and carriage. As a result, registration of colors can not be made at a predetermined position, resulting in misregistration of the colors. The misregistration of color may reduce the image quality.